1. Field of the Invention
The present invention relates to silicon carbide sintered bodies useful in the removal of diesel soot particles from the exhaust gas of diesel engines. The invention more particularly relates to a method for producing the silicon carbide sintered bodies, in which a raw material batch of a carbon precursor and silicon metal powder is combined with a water-soluble thermoplastic binder, an inorganic filler and optionally an organic fibrous filler.
2. Background and Discussion of the Related Art
Silicon carbide honeycomb structures are desirable for industrial and automotive applications. Silicon carbide honeycomb structures are particularly useful because of the very high surface area to volume ratio and the low-pressure drops associated with such structures in applications where high flow rate streams are to be treated. The high surface area allows for high loading levels and hence longer life in filtration applications. Furthermore, the high chemical inertness, very high refractoriness, high thermal conductivity and thermal cycling resistance make such SiC honeycombs particularly desirable for many high temperature filtration applications. These silicon carbide honeycomb structures are particularly useful in diesel particulate filtration for use in the removal of soot particles from the exhaust gas of diesel engines. Specifically, it is the very high thermal conductivity as well as the high heat capacity of the silicon carbide that functions to minimize hot-spots in the filters during regeneration, that makes SiC particularly useful in these diesel application.
In general, the method for forming such silicon carbide honeycomb structures involves fabrication via the formation of a green body of fine particulate material, which is thereafter sintered. The sintering is typically carried out at very high temperatures, normally greater than 2000xc2x0 C., for long periods of time because of the diffusive processes involved. Standard methods described in the literature typically involve extruding powdered silicon carbide with a thermoplastic binder, followed by burning off of the binder and thereafter sintering of the powder; see, e.g., U.S. Pat. No. 5,914,187. The drawback of the sintering method described is that very high temperatures (in excess of 2000xc2x0 C.) and long periods of xe2x80x9ctime at temperaturexe2x80x9d (6-10 hours) are needed for consolidation, making the process very expensive. Furthermore, large structures with uniform properties are difficult to form because the so-formed structure does not have sufficiently high strength after the binder is burned out (typically around 800xc2x0 C.), so as to be handled during the typical industrial process.
U.S. Pat. No. 5,853,444 (Maier) discloses a method for the production of a porous permeable molded body made of silicon carbide. The method disclosed therein involves utilizing a starting powder comprised of silicon, or a mixture of silicon with portions of silicon carbide and/or carbon. The starting powder is thereafter combined with an organic binding agent capable of being coked. This starting powder/binder mixture is then molded, specifically extruded, into a green body that is then subjected to an inert-gas atmosphere coking treatment. The molded body produced in this manner is then heated in the presence of nitrogen, or an inert gas containing nitrogen, to such a temperature sufficient to cause the free silicon to be converted with the carbon in a reaction to form silicon carbide; i.e., a recrystallization firing performed at greater than 2000xc2x0 C.
U.S. Pat. No. 5,707,567 (Pfaff) discloses a process for forming a self-sintered silicon carbide composite material. The particular steps for the formation involve mixing together an inert solvent and a raw material batch comprising the following: (1) about 50-90%, by weight silicon carbide; (2) about 5.0 to about 50% graphite particles coated with a carbon precursor; (3) about 2 to about 20% of sintering aids; and, (4) about 0.75 to about 15% by weight temporary filler. The method next involves drying the raw batch to evaporate the inert solvent and thereafter shaping the dried raw batch into a shaped body. After shaping the body into the desired shape, the method involves heating the shaped body at a temperature sufficient to carbonize the carbon precursor and volatilize the temporary filler thereby forming a matrix having interconnected pores; sufficient temperature disclosed for carbonizing the green body is 850xc2x0 C. Lastly, the method for forming the self-sintered silicon carbide composite material involves sintering the shaped body to densify the matrix; sintering temperatures ranging from 1900 to 2500xc2x0 C. are disclosed.
Both the Maier and Pfaff methods described are capable of use in forming silicon carbide bodies, however the disadvantage of using each of these methods is that, like most standard SiC sintering processes, they require extraordinarily high sintering temperatures, typically in excess of 2000xc2x0 C. Additionally, the Maier method results in the formation of very fragile after-carbonization structures, due to the very low carbon content material (i.e. starch or modified starch) used as the binder. Although the Pfaff method is suited to forming a graphite-containing silicon carbide, it is not suitable for the formation of ceramic bodies useful in filtration applications. Lastly, the batches disclosed in the Pfaff reference are not suitable for use in the extrusion process required for the formation of honeycomb structures.
Various SiC formation methods have been utilized involving reduced sintering temperatures, in an attempt to reduce both the cost and complexity of forming silicon carbide structures.
U.S. Pat. No. 5,196,389 (Dubots) describes the formation of silicon carbide supports. The method involves forming a plasticizable mixture of silicon powder, resin and carbon powder. The mixture is thereafter shaped, preferably by extrusion, to form a green monolithic honeycomb structure suitable for use as the catalyst support. Heating of this green monolithic body in non-oxidizing atmosphere at a temperature of between 600 and 1000xc2x0 C. is continued for a period of time sufficient to carbonize all of the resin in the mixture/green body. Lastly, the green monolithic honeycomb body is sintered, in a non-oxidizing (or nitriding) atmosphere, at temperature of between 1350 and 1450xc2x0 C. for a period of between 1 to 2 hours.
Although the sintering temperature disclosed in this SiC formation method are reduced when compared to those temperatures used in reaction sintering methods such as the Maier and Pfaff methods, there are several drawbacks associated with this formation method. The type of resins disclosed in this Dubots reference exhibit such a sufficiently high viscosity that the resins require dilution in ethanol in order to allow them to mix with a high percentage of fine solid powders. This solvent must then be removed during the heating and curing of the resin and thereafter recycled, or more practically burned off; as such this disclosed process requires unnecessary complexity and cost.
Dubots discloses the use of carbon black as the major carbon source; i.e., very fine soot powders with high surface area. Typically, carbon does not bond to any binder very well, the result being that structures produced with carbon-based fillers are typically weak. Furthermore, when the carburetion is carried out at 1400xc2x0 C., in order to form silicon carbide, the silicon powder has to melt and mix completely with the carbon. Discrete carbon particles make it difficult for this interaction to take place uniformly throughout the structure, thereby resulting in very small grained sintered SiC structures that are typically weak and non-uniform. Although the strength and uniformity of the structure may be sufficient for thick-walled catalyst support structures, this strength is not sufficient for thin-walled, controlled porosity honeycomb structures, where the efficiency of the filtration is controlled by the wall properties; i.e., diesel particulate filtration applications.
U.S. Pat. No. 5,217,930 (Dubots) discloses a porous refractory carbide with a skeleton of metal or metalloid refractory carbide for catalyst support. Particularly, the reference describes a method for the formation of silicon oxycarbide coated on metallic or SiC-type porous substrate. The method of forming this silicon oxycarbide material involves mixing an organic polymeric or polymerisable compound with metal powder and a solvent and thereafter curing the formed mass after removal the solvents. Heat treating at 500-1000xc2x0 C. is then utilized for forming the porous skeleton; i.e., carbonization. After carbonization the structure is subjected to xe2x80x9cactivationxe2x80x9d i.e., partial or controlled oxidation in presence of oxygen (500-900xc2x0 C.). After oxidation, heat treatment, in inert atmosphere, is carried out at temperature not exceeding 1400xc2x0 C., preferably 1150-1250xc2x0 C., to form an amorphous SiOxCy compound, while part of the structure forms silicon carbide.
Although the sintering temperatures utilized in this method are reduced over those disclosed in the aforementioned processes, this method is not suitable for use in extruding thin-walled SiC honeycomb structures. Specifically, as was the case in the previous Dubots reference, the high viscosity of the resin utilized (e.g., as high as 4000 cp), results in a high degree of oligomer reaction, thereby rendering the resin sparingly, if at all, soluble in water. As a result of this, few reactive sites are available, and thus the resin cannot be diluted with water, thereby resulting in the severe splitting of walls, and in turn leading to the formation of weak silicon carbide honeycombs.
There is, accordingly, a clear need for, and thus an object of the present invention is to provide, for a reduced sintering temperature process suitable for use in the formation of extruded thin-walled honeycomb silicon carbide structures.
Accordingly, the object of the present invention is directed at both a raw material batch mixture and a process for forming green and lowered-temperature sintered SiC honeycomb structures that exhibit with sufficient handling and so-formed strength.
Specifically, the invention is directed at a plasticizable raw material batch mixture for forming a silicon carbide honeycomb structure, comprising the following components: (1) powdered silicon metal; (2) a carbon precursor comprising a water soluble crosslinking thermoset resin having a viscosity of less than about 1000 centipoise (cp), and preferably less than about 500 cp; (3) a powdered silicon-containing filler; and, (4) a water soluble thermoplastic binder. Optionally, the batch mixture can include either, or both, an organic fibrous filler and a pore forming filler comprising either a graphitic or a thermoplastic pore-forming filler.
In another aspect, the invention provides for a process for producing a silicon carbide honeycomb structure utilizing the inventive batch material. The process involves the following steps: (1) mixing together selected raw materials to form the previously mentioned plasticizable raw material batch; (2) shaping the batch mixture to form a shaped green body, preferably involving extrusion to form a honeycomb structure; (3) drying and curing the green body; (4) heating the green body in a nitrogen atmosphere at a temperature sufficient to carbonize the resin; and, (5) sintering the green body at a temperature sufficient to convert the green body to a porous silicon carbide sintered body; preferably a temperature at least above 1400xc2x0 C.